1. Field of the Invention
The present invention relates to the manufacturing of optical lenses and particularly of lenses for CMOS imagers.
2. Description of the Related Art
Imagers produced according to the CMOS (“Complementary Metal Oxide Semiconductor”) technology are currently the subject of an increasing number of applications due to their low cost price in comparison with CCD (Charge Coupled Device) imagers. Such CMOS imagers were initially used to produce low resolution image sensors of mediocre quality (for example web cameras). Today, after major investment in research and development, CMOS imagers can compete with CCD imagers. The present invention is in line with an effort to improve this imager technology aiming to reduce the cost with the same quality.
FIG. 1 represents an example of a module for capturing images and/or video using a CMOS imager, intended for example to be mounted into a portable device such as a mobile telephone, a camera or a video camera. The module 1 comprises a frame 2, an optical set or lens-holder block 3, a lens 4 fitted into the block 3, an infrared filter 5 and a base 6. A CMOS imager 10 is arranged on the base 6 and receives the light passing through the microlenses and the infrared filter.
The CMOS imager 10 is in the shape of a semiconductor chip and comprises a plurality of photosites each forming one pixel (not visible in FIG. 1). Each pixel comprises a photodiode and a control and interconnection circuit of the photodiode. The pixels are arranged as an array and a mosaic of red, green and blue filters is distributed over the pixel array, generally according to the Bayer architecture (the cells of a line being alternately red and green, or alternately green and blue). Each pixel is thus covered with a determined, red, green or blue, primary color filter and provides a piece of luminance information about the primary color allocated to it, forming a piece of pixel information.
FIG. 2 is a schematic cross-section of the CMOS imager 10 in a region corresponding to three pixels PIX1, PIX2, PIX3. Going from bottom to top, layers 11, 12, 13, 14, 15 and microlenses L0 (L0-1, L0-2, L0-3) can be distinguished. The layer 11 is the semi-conductive substrate on which the imager is implanted. This layer 11 thus represents the active part of the imager and comprises photodiodes and their associated control and interconnection circuits (not detailed). The layer 12 is formed by a dielectric material that entirely covers the substrate 11. The layer 13 is a passivating layer deposited on the imager at the end of the CMOS manufacturing process. The layer 14 is formed by colored resins and comprises red, green or blue areas 14-1, 14-2, 14-3 forming the above-mentioned primary color filters, with one color filter per pixel. The layer 15 is an intermediate layer of glass or resin forming a base for the microlenses L0 and providing good flatness. The microlenses L0 are arranged in a so-called “MLA” (“Microlens Array”) with one microlens per pixel.
The lens 4 of the optical set is generally formed in a mold by means of a polymer resin that is removed from the mold after a baking step. Another known technique of manufacturing the lens 4 involves printing polymer resin on a base, then the resin creeping to obtain a convex (rounded) face.
The microlenses L0 are also manufactured by means of a polymer resin and the method for manufacturing the latter comprises for example steps of depositing a layer of light-sensitive polymer resin (photoresist) onto an imager wafer, a step of soft bake, a step of exposing the layer of resin to an ultraviolet light through an isolation mask and a step of removing the insolated parts with an organic solvent to obtain an array of flat lands. The flat lenses are then subjected to a determined temperature to undergo a thermal creep that renders their upper face convex. They are then hardened by a final operation of annealing.
These manufacturing methods and particularly the method for manufacturing the lens of the optical set using a mold, may be complex, lengthy to implement and costly as a result.